The invention relates to a circuit arrangement for decoding digitally coded frequency-modulated signals, and more particularly to a circuit for detecting and eliminating noise induced pulse signals at the output stage of the frequency demodulator.
During FM transmission analysis of the signal after demodulation reveals the existence of an operating threshold. Defined in terms of carrier-to-noise ratio, this threshold is generally about C/N=10 dB. If this ratio is higher than the above-mentioned limit, the perturbation affecting the output signal of the frequency demodulator is only constituted by a continuous noise whose spectral power density is substantially proportional to the square value of the frequency if the possible de-emphasis function is ignored.
If, on the other hand, this threshold is not reached, the original restored signal is not only perturbed by the continuous noise but also by noise of a pulsatory type. This type of perturbation constituted by signals referred to as clicks is characterized by the appearance of very short pulses having a large energy, an arbitrary amplitude and an equally arbitrary temporal position. The frequency at which these pulses occur is larger as the said C/N ratio is smaller.
Referring to FIGS. 1a and 1b the presence of these clicks can be approximately interpreted as follows. Assuming that the modulating signal is zero, the carrier A is fixed and the noise vector b(t) turns arbitrarily around the extremity of the carrier, as is shown in FIG. 1a. In reality the frequency demodulator supplies at its output a signal which is proportional to the derivative of the angle .phi.(t) if the de-emphasis is not taken into account. Thus, if the carrier-to-noise ratio at the input of the receiver is large, as in the case of FIG. 1a, the noise amplitude is small with respect to that of the carrier and the angle .phi.(t) fluctuates around an average value of zero. The output signal d.phi.(t)/dt of the demodulator thus has small variations around the value 0.
However, if the noise increases, the amplitude of the noise vector may be as large or even larger than that of the carrier at certain moments. In this situation, shown in FIG. 1b, the resultant vector R(t)=A+b(t) may turn around the origin. Under these circumstances the angle .phi.(t) varies abruptly by .+-.2.pi. and the output signal which is proportional to d.phi./dt is constituted by a short pulse having a very large amplitude and referred to as click as mentioned above. This type of noise deviates considerably from the white noise model.
If the transmission signals are digitally coded, it is of course necessary to use a digital decoder after the frequency demodulator at the receiving end. If the clicks appear in an arbitrary manner, they may pass the signal from the level at which it is present to another coding level and thereby introduce errors during decoding. The smaller the carrier/noise ratio, the more numerous the clicks and the more frequently these decoding errors occur.
It is thus undeniable that the conventional decoders such as, for example, threshold decoders or the more recent maximum likelihood decoders are better adapted to operate in an environment in which the noise is white, additive and Gaussian than in the case of FM transmission, particularly at a low carrier level. Experiments carried out in the case of satellite transmission of a television signal confirm this fact. The sound part in such a signal is digitally and duo-binary coded and the satellite transmission uses frequency modulation. A detailed analysis of the errors made by the one or the other type of decoder in this field of application confirms that these two decoders almost systematically produce a decoding error when a click occurs, resulting in a degradation of performance of these decoders during FM transmission.
It is an object of the invention to provide a technical solution yielding a better performance during frequency modulation transmission.